This invention relates to an improved process for preparing aromatic fluorides by diazotization-fluorination in hydrogen fluoride.
Preparation of aromatic fluorides from corresponding aromatic primary amines by diazotization-fluorination processes is well known. In such processes, (A) an aromatic primary amine is diazotized in a reaction mixture comprising (i) a reaction medium consisting essentially of hydrogen fluoride, (ii) the amine and (iii) a nitrosonium ion-containing or -generating diazotization agent under reaction conditions such that the resulting reaction mixture comprising the resulting aromatic diazonium fluoride typically further comprises one or more oxidizing agents, e.g. nitrous acid or other nitrosonium ion-containing or -generating composition, whereby the reaction mixture has a positive oxidation potential as shown by its production of oxidant-indicating blue color on test paper impregnated with starch and potassium iodide, and (B) the aromatic diazonium fluoride is decomposed at elevated temperature in the resulting reaction mixture to nitrogen and the aromatic fluoride.
The presence of oxidizing agent, such as excess nitrosonium ion, in the reaction mixture wherein the diazonium fluoride is being decomposed results in substantially increased corrosion, low yields of the desired aromatic fluoride, and formation of undesired byproducts such as resinous material and tar. The presence of oxidizing agent in the reaction mixture results from, for example, charging a nitrosonium ion-containing or -generating diazotization agent in an amount in excess of the amount stoichiometrically required for diazotization of the entire amount of aromatic primary amine employed. Such an excess is often intentionally employed to maximize the amount of amine converted to diazonium fluoride. In other instances, such excess per se is not intentionally employed; rather, it is desired merely to convert the entire amount of the amine, but due either to loss of some of the diazotization agent (e.g., loss of some nitrous acid as NO or NO.sub.2) or one or more other reasons it is necessary to test the reaction mixture as with starch-potassium iodide paper to determine whether a sufficient amount of the diazotization agent has been added. Since such test merely shows the presence or absence of oxidizing agent (e.g. nitrous acid), additional diazotization agent (e.g. sodium nitrite) is added until the starch-potassium iodide test shows the characteristic oxidant-indicating blue color, corresponding to at least a slight excess of diazotization agent.
Preparation of diazonium salts by reaction of primary aromatic amines with nitrous acid is discussed by Morrison and Boyd, Organic Chemistry, 1959, pages 570-571. According to Morrison and Boyd, nitrous acid is generated in the presence of the amine by the reaction between sodium nitrite and a mineral acid, "usually" hydrochloric acid or sulfuric acid; diazotization is "generally" carried out in a way wherein the amine is dissolved or suspended in an "aqueous solution" of the mineral acid and excess sodium nitrite is added, thereby generating excess nitrous acid which interferes with subsequent reactions of the diazonium salt; and accordingly the excess nitrous acid is "destroyed" by the addition of urea, which reacts with nitrous acid to form nitrogen, carbon dioxide and water.
Unfortunately, in the course of attempting to solve the problems which are substantially overcome by this invention, addition of urea to a diazotized reaction mixture containing at least one nitrosonium-ion based oxidizing agent in a reaction medium consisting essentially of hydrogen fluoride was found to be ineffective for destroying the oxidizing agent. Other agents which proved to be ineffective for destroying the oxidizing agent are hydroxylamine and its salts, metabisulfides and iron compounds. Sulfamic acid, an agent which has largely supplanted the use of urea as a NO.sup.+ quencher in aqueous systems, was found to be ineffective in HF due to its poor solubility and resulting slow reaction rate. Phosphorous chloride was found to be an effective agent, but results in unwanted chloride-containing by-products such as HCl. The present invention effectively solves the problems associated with destroying the oxidizing agent in diazotized reaction mixtures without resulting in unwanted by-products.
Various compounds which were thought to be nitrous acid scavengers were compared in Fitzpatrick et al., J. Chem. Soc. Perkin Trans. II p. 927-932 (1984), including urea, hydroxylamine, hydrazoic acid, hydrazine, sulphamic acid, and 4-nitroaniline. The tests were conducted over a range of acidity of 0-1.5M HClO.sub.4 to determine which compounds were the best nitrous acid scavengers. The acidity levels tested in the Fitzpatrick article are about a magnitude of about 10.sup.8 -10.sup.9 weaker in acidity than the HF solution of a diazotization reaction. It has been found, therefore, that the results of these tests do not help in determining effective nitrous acid scavengers for these reactions. For example, as noted above, urea and hydroxylamine hydrochloride were found to be ineffective and did not react with dilute NO.sup.+ in HF. Sulfamic acid powder reacted slowly because of its lack of solubility in HF. Hydrazoic acid, which was listed in the Fitzpatrick article as one of the best nitrous acid scavengers at the acidity levels tested, is very toxic and decomposes at the high acidities found in HF solutions used in diazotization-fluorination reactions. Hydrazinium ion, while listed as a nitrous acid scavenger, is only sparingly soluble in HF. Using nitroanilines, as suggested by the article, complicates the diazotization-fluorination process since the resulting diazonium salts decompose at much higher temperatures, resulting in a toxic, potentially explosive waste. Also, excess nitroaniline that is undiazotized must be eliminated from the HF residue because of its toxicity and established Environmental Protection Agency limits.
Therefore, the problem of destroying excess nitrous acid in HF solution could not be solved by the use of traditional nitrous acid scavengers, since the reaction of these scavengers is unpredictable in the high acidity levels present.